With recent implementation of blue LED in practical use, studies are being aggressively made to develop white LED by utilizing the blue LED. The white LED is low in power consumption and extended in life as compared with existing white light sources and therefore, its application to a liquid crystal panel backlight, an indoor or an outdoor illumination device, and the like, is proceeding.
The white LED developed at present is obtained by coating Ce-doped YAG (yttrium.aluminum.garnet) on the surface of a blue LED. However, the Ce-doped YAG has a photoluminescent wavelength in the vicinity of 530 nm and when this photoluminescent light and the light of blue LED are mixed to provide white light, the resulting color of the light is slightly blue-tinted and good white light cannot be obtained.
On the other hand, a sialon-based phosphor activated with a rare earth element is known to emit photoluminescent light with a longer wavelength (shifted to the red side) than the photoluminescent wavelength of Ce-doped YAG (see, Japanese Unexamined Patent Publication (Kokai) No. 2002-363554). When this photoluminescent light of sialon-based phosphor is mixed with the light of blue LED, good white light can be obtained. In this way, practical use of a sialon-based phosphor material as a new phosphor material is expected.
However, the phosphor having the composition disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-363554 uses crystalline silicon nitride as the raw material and is produced by a hot-pressing method where the reaction into sialon is allowed to proceed while compressing the powder material. The sialon-based phosphor obtained by this method is in the lump form and is not suited for use as a phosphor powder.
The present inventors had attempted to obtain a sialon-based phosphor by mixing a crystalline silicon nitride powder with a calcium source, a rare earth element source and an aluminum source and firing it in an atmospheric pressure furnace. As a result, it was confirmed that a phosphor in the powder state can be obtained by selecting the appropriate particle size of silicon nitride and the reaction conditions, but the powder is a gathering of strongly agglomerated particles (hereinafter called a “agglomerated particles”) and the agglomerated particles have a large diameter and a broad and non-uniform particle size distribution. A phosphor comprising such agglomerated particles is improper for forming a thin film by mixing it with a resin and also, the photoluminescent light emitted therefrom is non-uniform and insufficient in the photoluminescent intensity.
The present invention has been made to solve these problems and an object of the present invention is to provide a method for producing a sialon-based phosphor not having a broad size distribution. Another object of the present invention is to provide a sialon-based phosphor, obtained by the method, which can form a thin film and give uniform photoluminescent light with high light emission intensity.